Polyacetal resins are excellent in balance of mechanical properties, chemical resistance and coefficient of friction, and therefore, have been used as typical engineering plastics over a wide range including electric and electronic parts, automobile parts and other mechanical parts. When so used, thermal aging resistance and surface appearance of molded article are important characteristics.
Polyacetal resin compositions prepared by adding an amino-substituted triazine compound to a polyacetal resin are already known. For example, Japanese Patent Application Kokai No. 60-90,250, Japanese Patent Application Kokai No. 7-173,368 and Japanese Patent Application No. 7-331,028 disclose polyacetal resin compositions for the purpose of improving the thermal stability. Also, Japanese Patent Application Kokai No. 2-80,416 discloses a method of the addition of the above compound to a crude polyacetal resin before the terminal stabilization for the purpose of enhancing the compatibility with the polyacetal resin.
However, in these methods, the amino-substituted triazine compound is poor in compatibility with a polyacetal resin, and hence, cannot be dispersed in the form of particles having an average particle size of not more than 1 .mu.m in the polyacetal resin. In addition, when the molded article is allowed to stand for several days, the compound bleeds to the surface of the molded article resulting in bad surface appearance. On the other hand, Japanese Patent Application Kokai No. 2-80,416 discloses a method of improving the non-bleeding properties of an amino-substituted triazine compound. However, by this method, it is impossible to obtain a polyacetal resin molded article in which the amino-substituted triazine compound is dispersed in the form of particles having an average particle size of not more than 1 .mu.m in the polyacetal resin. This is because the time of melt-mixing the polyacetal resin with melamine is long and hence formaldehyde reacts with the melamine and the reaction products further react with each other and aggregate, whereby the particle sizes of dispersed particles become larger.